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The most important diagnostic units used to optimize the FEL beam are marked.
The detectors measure the intensity, position, spectrum, spatial profile, and arrival time of the FEL pulses.
Layout of the User Facility
The photon beam transport system delivers the FEL pulses to one out of five experimental stations at a time. The beam is switched between stations by remotely controlled plane grazing incidence mirrors.
- The direct “non-monochromatized” beam is delivered to the beamlines BL1, BL2 and BL3. The beamlines PG1 and PG2 are equipped with a high-resolution monochromator selecting a narrow spectrum of the FEL pulse.
- The optical laser in the laser hutch is used as a pump source for femtosecond time-resolved pump-and-probe experiments where usually the FEL pulse acts as the probe. The synchronization of these extremely fast pulses is checked by a Timing Electro-Optical sampling (TEO) system placed in the linear accelerator, and by a streak camera that measures the arrival time of the optical laser pulse and of the synchrotron radiation generated by the electron bunches when they are deflected via a dipole magnet into the dump.
- The intensity of the FEL pulses varies due to the stochastic nature of the SASE process. Non-invasive measurements of the intensity of individual pulses are performed by four Gas Monitor Detectors (GMD) that also determine the position of each pulse during experiments. The gas monitor detectors are located at the end of the accelerator tunnel and the beginning of the experimental hall.
- The fluctuations in wavelength – within the FEL bandwidth of approximately 1 percent – from pulse to pulse are specific for the SASE process. Single-shot spectra can be measured by a new Variable Line Spacing grating spectrometer (VLS).
Spatial profile of the FEL beam on the Ce:YAG crystal averaged over 3 bunches. The laser-engraved cross is clearly visible. The average energy in the radiation pulse was 40 μJ at a wavelength of 13 nm.
Ensuring the Pointing Stability
When setting up the machine after maintenance operations or upgrades, it is crucial to ensure pointing stability of the FEL beam and optimum focusing at the experimental stations. For this purpose a Ce:YAG fluorescent crystal with a laser-engraved cross is incorporated in detector unit 2 in the tunnel. Centering the beam on this cross ensures that it can accurately propagate across all mirrors towards the experiments.
The diagram demonstrates the performance of the attenuator. The blue curve shows the transmission signal in real time, while the green curve displays a time average with a time constant of 30 minutes.
The Gas-filled Attenuator
The extremely intense FEL pulses from FLASH may destroy a sample in a single shot. Thus, for aligning delicate samples or determining the intensity dependence of processes without changing the beam characteristics, a windowless gas-filled cell with differential pumping units can be used. The 15-m-long gas-filled attenuator is placed in front of the experimental hall between the two pairs of gas monitor detectors. The maximum gas pressure is about 0.1 mbar. Nitrogen covers a sufficient attenuation range of at least five orders of magnitude in the spectral range of 19 to 60 nm. Between 6.5 and 19 nm xenon and krypton can be used.
